If you make a small change in your formula, it should work so that you can simply fill your formula down 500 rows or more..

Instead of =AVERAGE(A1:A500)

Use =AVERAGE(A1:$A$500)

Once you have entered this into B1, click on the "fill handle" in the lower right corner of the cell and drag down to row 500. Your formula will update, but the $A$500 part will not change. It will end up like:

=AVERAGE(A2:$A$500)
=AVERAGE(A3:$A$500)
=AVERAGE(A4:$A$500)...etc, down to
=AVERAGE(A500:$A$500)